An accepted treatment for cataracts is surgical removal of the lens and replacement of the lens function by an artificial intraocular lens. In the United States, the majority of cataractous lenses are removed by a surgical technique called phacoemulsification. Prior to removing the cataractous lens, an opening, or rhexis, must be made in the anterior capsule. During phacoemulsification, there is a great deal of tension on the cut edges of the anterior capsulorhexis while the lens nucleus is emulsified. Accordingly, a continuous cut or tear (rhexis), without “tags,” is a critical step in a safe and effective phacoemulsification procedure.
If the capsule is opened with numerous small capsular tears, the small tags that remain can lead to radial capsular tears which may extend into the posterior capsule. Such a radial tear constitutes a complication since it destabilizes the lens for further cataract removal and safe intraocular lens placement within the lens capsule later in the operation. Further, if the posterior capsule is punctured then the vitreous may gain access to the anterior chamber of the eye. If this happens, the vitreous must be removed by an additional procedure with special instruments. The loss of vitreous is also associated with an increased rate of subsequent retinal detachment and/or infection within the eye. Importantly, these complications are potentially blinding.
Conventional equipment used for phacoemulsification includes an ultrasonically driven handpiece with an attached cutting tip. In some of these handpieces, the operative part is a centrally located, hollow resonating bar or horn directly attached to a set of piezoelectric crystals. The crystals supply ultrasonic vibration for driving both the horn and the attached cutting tip during phacoemulsification.
Many of the known devices and methods used for the capsulorhexis procedure require a great deal of skill on the part of the surgeon to produce a continuous curvilinear capsular opening. This is due to the extreme difficulty in controlling the path of the cutting tip of the device. For example, a typical procedure begins with a capsular incision made with a cystotome, e.g., a cutting tip as described above. This incision is then coaxed into a circular or oval shape by pushing the leading edge of the incision in the capsule, using the cystotome as a wedge rather than in a cutting fashion. Alternatively, the initial capsular incision may be torn into a circular shape by grasping the leading edge with fine caliber forceps and advancing the cut. Either of these approaches involves a very challenging maneuver and the tearing motion can sometimes lead to an undesirable tear of the capsule toward the back of the lens, even in the most experienced hands.
Moreover, even if a smooth capsular opening without tags is ultimately produced, the size and/or position of the capsular opening may present a problem. For instance, a capsular opening that is too small can impede the safe removal of the lens nucleus and cortex and prevent proper intraocular lens insertion into the lens capsule. The additional stresses necessary to accomplish the operation with a small or misplaced capsular opening put the eye at risk for zonular and capsular breakage. Either of these complications will likely increase the length and complexity of the operation and may result in vitreous loss.
A continuous, properly positioned, and circular opening is thus highly desirable because it results in: (1) a significant reduction in radial tears and tags within the anterior capsule, (2) capsule integrity necessary for proper centering of a lens prosthesis; (3) safe and effective hydrodissection; and (4) safe use of capsular procedures on patients having poorly visualized capsules and/or small pupil openings. In addition, the capsulorhexis should be properly dimensioned relative to the diameter of the intraocular lens being implanted in order to reduce the chances of a secondary cataract, also called posterior capsule opacification and for use with proposed accommodative intraocular lens designs. Therefore, there is a continuing need for improved devices for performing anterior chamber capsulorhexis.
Various methods and devices for automating the capsulorhexis process have been proposed. One approach is described in U.S. patent application Ser. No. 12/618,805, filed 16 Nov. 2009 and titled “Capuslarhexis Device Using Pulsed Electric Fields” (hereinafter “the '805 application”). The '805 application, the entire contents of which are incorporated by reference herein, describes methods and apparatus for performing capsulorhexis using high-frequency electrical currents applied to the anterior lens capsule through a unipolar electrode. The device uses pulsed electric fields to perform the cutting action—the pulsed electric field is generated using a ring electrode, placed against the anterior capsule of the eye, and a grounding electrode located at a different position inside or outside the eye. In some embodiments of this system, the ring electrode comprises a thin, electrically conducting wire. A very small cross-section (e.g., less than about 0.25 millimeters in diameter) will yield high-intensity electric fields close to the wire; these electric fields will reduce in intensity further away from the wire. Because a ground electrode having a much larger cross-section than the cutting electrode is used in this system, the electric fields remain attenuated at the grounding electrode, and a high proportion of the available cutting energy is deposited into a thin region immediately around the cutting electrode's wire.
Another system is described in United States Patent Application Publication No. 2006/0100617, the entire contents of which are incorporated herein by reference. This publication describes an autocapsulorhexis device comprising a circular, flexible ring made of an elastomer or an acrylic or thermoplastic material. Embedded within each of various embodiments of this flexible ring is either a resistance-heating element or a pair of bipolar electrodes, which are energized according to known techniques to produce localized heating on the anterior capsule, so as to define a weakened boundary for an easy detachment of the portion of the capsule within the circular ring. Various other devices have been proposed, many of which depend on resistive-heating cautery elements, such as U.S. Pat. No. 6,066,138, issued May 23, 2000; U.S. Pat. No. 4,481,948, issued Nov. 13, 1984; and WIPO Publication No. WO 2006/109290 A2, published Oct. 19, 2006. The entire contents of each of the references identified in this paragraph are incorporated by reference herein, for the purpose of providing background and context for the present invention.